Omega-Conus geographus toxin (omega CgTX), a peptide from the venom of a marine snail, blocks specific calcium channels in neurons. Omega CgTX acts on the cell's external surface at a site which is either the Ca channel itself or a closely associated component. Omega CgTX also blocks depolarization-evoked release of transmitter at synapses of all lower vertebrates tested. Omega CgTX blocks frog synapses irreversibly, however, it can be chemically modified to act reversibly. Omega-Conus magus toxin (omega CmTX), a peptide with a sequence somewhat homologous to omega CgTX, behaves similarly to omega CgTX. The proposed research will use omega CgTX, omega CmTX, chemically derivatized toxins and antitoxin antibodies to probe synaptic Ca channels. There are three primary aims. 1. Examine the hypothesis that the presynaptic terminal at the endplate of frogs has, in addition to toxin-sensitive Ca channels, toxin-insensitive Ca channels. The involvement of these two classes of Ca channels in transmitter release as well as their pharmacological properties, will be studied with conventional intra- and extracellular recording techniques. 2. Examine the hypothesis that certain Na and Ca channels may be structurally related. A fraction of the Na current in cultured DRG neurons from chick is also blocked by omega CgTX suggesting that certain Na and Ca channels may share a common binding site for omega CgTX. The susceptibilities of Na and Ca currents to different omega-toxins and toxin derivatives will be examined with whole-cell voltage clamping. These experiments are also expected to provide information concerning structure- function relationships of the toxins. 3. Examine the hypothesis that synaptic Ca channels are confined to the sites of transmitter release. This will be done by mapping, at the level of the light microscope, the distribution of omega CgTX binding sites on the presynaptic terminals at endplates of frogs. Several approaches, including immunohistochemistry with anti-omega CgTX antibodies, will be used. Our long-range goal is to characterize in detail the cellular and molecular biology of synaptic Ca channels and trace the changes in their structures in the course of ontogeny and evolution. Such a study should provide valuable insight into structure-function interrelationships, the regulation of synaptic Ca channels, and perhaps the role the channels may play in synaptic plasticity.